The Puppet Masters (1951) by Robert A Heinlein

Wikipedia wrote:The Puppet Masters is a 1951 science fiction novel by Robert A. Heinlein in which American secret agents battle parasitic invaders from outer space. The novel was originally serialised in Galaxy Science Fiction (September, October, November 1951).

The book evokes a sense of paranoia later captured in the 1956 film Invasion of the Body Snatchers, which had a similar premise. Heinlein's novel also repeatedly makes explicit the analogy between the mind-controlling parasites and the Communist Russians, echoing the then prevailing Second Red Scare in the United States.

Wikipdeia wrote:The setting is the early 21st century (the first scene is in 2007). There had been a nuclear war between the Soviet Union and the West which left both sides battered but unbroken, and following the hot war they just went back to the Cold War. The Soviet Union and China remain a single bloc dominated by Moscow, and the sharp Sino-Soviet breach of the late 1950s had not happened. (A casual reference to Kurdish as one of the languages which an agent "behind the curtain" finds useful indicates that the Soviets have at some time taken over some Kurdish-speaking areas.

Social customs have changed somewhat, in a way typical for Heinlein's fiction (i.e. having become more liberal, such as marriage contracts being possible with fixed terms etc.) and rayguns and personal flying cars are commonplace. Space stations exist and colonies have been established on the planet Venus. Space technology is far more advanced than in the actual first decade of the 21st century. For example, in the last scene, a space warship is sent on a twelve-year trip to Titan, with not only life-support for a large crew but also enough armaments - presumably nuclear - to all by itself confront an entire world.

Plot
In the summer of 2007 Earth is under clandestine attack. Slug-like creatures, arriving in flying saucers, are attaching themselves to people’s backs, taking control of their victims’ nervous systems, and manipulating those people as puppets. The Old Man, the head of clandestine national security agency called the Section, goes to Des Moines, Iowa, with Sam and Mary, two of his best agents, to investigate a flying saucer report, but much more seriously the ominous disappearance of the six agents sent previously. They discover that the slugs are steadily taking over Des Moines, but they cannot convince the President to declare an emergency.